Magnetohydrodynamic generator



.51 0-11 SR SEARCH RDM FIFEKSOZ 53R 3l82213 y 4,1965 R. J. RosA-3,182,213

MAGNETOHYDRODYNAMIC GENERATOR Fil une 1. 1961 4 Sheets-Sheet LOA DPIRRIOR ART GENERATOR l +ls RICHARD J. ROSA INVENTOR.

QMWDW WMKQM ATTORNEYS May 4, 1965 J. ROSA MAGNETOHYDRODYNAMIC GENERATOR4 Sheets-Sheet 2 Filed June 1, 1961 TIME EQUILIBRIUM VALUE TIME RICHARDJ. ROSA IN V EN TOR MZ/wD/P ATTORNEYS y 4, 1965 R. J. ROSA 3,182,213

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ATTORNEYS y t, RICHARD J. ROSA United States Patent 3,182,213MAGNETOHYDRODYNAMIC GENERATOR Richard J. Rosa, Reading, Mass., assignorto Avco Corporation, Cincinnati, Ohio, a corporation of Delaware FiledJune 1, 1961, Ser. No. 114,120 30 Claims. (Cl. 310-11) The presentinvention relates to a means for and method of utilizing the Hall effectin electrical equipment and, more particularly, in magnetohydrodynamic(hereinafter abbreviated MHD) generators. For convenience, the presentinvention is described with particular reference to MHD generators,which generate power by movement of electrically conductive fluidrelative to a magnetic field, but is not limited to such applications.

MHD generators usually employ an electrically conductive working fluidfrom a high temperature, high pressure source. From the source, thefluid flows through the generator with which is associated a magneticfield and electrodes between which a flow of current is induced bymovement of the fluid relative to the field. The fluid exhausts to asink, which may simply be the atmosphere, or in more sophisticatedsystems, may comprise a recovery system including pumping means forreturning the fluid to the source. The working fluid may comprise a hightemperature, high pressure gas, such as helium or argon, to which isadded about 1% sodium, potassium or cesium to promote ionization andhence electrical conductivity. The gas is composed of electrons,positive ions, neutral atoms, and sub-atomic particles and may, forconvenience, be termed plasma.

If there is a current flow through a material or plasma perpendicular toa magnetic field, an electric field will be generated which isperpendicular both to the current and the field. This phenomenon, calledthe Hall effect, arises because of the force of the magnetic field on amoving charge. Such an electric field is commonly referred to as a Hallpotential, and the current flow resulting from the Hall potential iscommonly referred to as a Hall current. Thus, as plasma flows throughthe generator in the presence of an electric field and a magnetic fieldoriented at right angles to the electric field, curved movements ofcharged particles occur under the influence of both fields. By virtue ofsuch movements, separation of negative and positive charges occurs inthe plasma, resulting in a substantial potential gradient, or Hallpotential, along the length of its flow. Under the influence of the Hallpotential, Hall currents may circulate longitudinally through the plasmaif a closed circuit is otherwise available. The Hall currents opposedirect flow of current through the plasma between the electrodes andconstitute a serious loss of operating efficiency.

The idea of using the Hall potential in a generator in itself is notbroadly new. The Karlovitz et al. Patent, 2,210,918, which issued onAugust 13, 1940, entitled Process for the Conversion of Energy inApparatus for Carrying Out the Process, describes an early form of Hallcurrent generator. In patent application Serial Number 860,973, filedDecember 21, 1959, of which I am a co-inventor, there is described anMHD generator utilizing pairs of oppositely disposed, segmentedelectrodes which may be connected to separate loads for preventing theflow of Hall currents in the generator. Because of the fact that nocontinuous path is provided longitudinally through the electrodesparallel to the direction of plasma flow, Hall currents cannot formwithin the plasma. The segmentation of the electrodes in eflectcompletely breaks the path of Hall current flow. In this way, lossesassociated with Hall currents are eliminated and improved over-alloperation is obtained. In my patent application Serial Number 18,033,filed March 28, 1960, there is described an entirely differentarrangement 3,182,213 Patented May 4, 1965 comprising an improved Hallcurrent generator having opposed pairs of electrodes wherein the loadcircuit is connected between the terminal electrodes, i.e., the firstand last electrodes along the length of the generator duct. Each pair ofelectrodes intermediate the terminal electrodes are electricallyinterconnected in a specified manner to increase the Hall potential.

From the preceding discussion it may readily be seen that although theexistence of a Hall potential has been utilized in special cases togenerate current, the existence of Hall currents has heretofore beenconsidered undesirable, particularly in the usual form of MHD generator.

Thus, the prior art teaches that MHD generators must be designed tocreate a Hall potential whereby the current flow resulting therefrom maybe supplied to a load, or in the alternative, that the flow of Hallcurrents be prevented in all cases and that the current flow resultingfrom conventional MHD generator action be supplied to a load.

The present invention is based on the concept that the power output ofan electrical generator may be simply and advantageously controlled, asdistinguished from creating or generating the power output, by utilizingHall currents in a manner contrary to the teaching of the prior art. Anexample of my utilization of Hall currents contrary to the teaching ofthe prior art is the provision of an MHD generator wherein Hall currentsare selectively prevented and permitted to flow.

One embodiment of the present invention is directed to means for andmethod of controlling the output of the aforementioned types ofgenerators, and another embodiment is directed to means for and methodof generating alternating current in an MHD generator.

Briefly described, a novel MHD generator in accordance with the presentinvention comprises a duct and a magnetic field normal to the axis ofthe duct. Movement of the plasma through the duct and the field inducesan electromotive force between opposed electrodes that areinterconnected to accommodate circulation of current (conductioncurrent) transversely of both the magnetic field and the direction ofplasma flow. Switching means is associated with the terminal electrodesto permit circulation of Hall current longitudinally through the plasma.To facilitate identification of this switching means, it will be termed,for convenience, a Hall current switch or switches. Additional switchingmeans is also associated with the electrodes intermediate the terminalelectrodes to interrupt current flow between these electrodes. Tofacilitate identification of this additional switching means, it will betermed, for convenience, a load switch or switches. If three or more ofthe intermediate elec trodes are serially interconnected, one loadswitch connected in series with these electrodes may be suflicient. Ifpairs of oppositely disposed electrodes intermediate the terminalelectrodes supply separate loads, a load switch may be connected inseries with each load. Thus, assume that pairs of oppositely disposedelectrodes intermediate the terminal electrodes supply separate loads,the load switches associated with the loads are closed and the Hallcurrent switch associated with the terminal electrodes is open, i.e.,full load current is being delivered to separate loads by an MHDgenerator with segmented electrodes, and the flow of Hall currents isprevented. When the Hall current switch is closed, Hall current, whichcould not otherwise flow, is now permitted to flow longitudinallythrough the generator. The flow of Hall current reduces the conductioncurrent or current normal to the gas flow and coupled to the loads. Byreason of the reduction in the conduction current flowing through theloads, the load switches may now be opened with little or no arcing.Opening of the load switches will reduce the conduction current to zero.Reduction of the conduction current to zero in turn reduces the Hallcurrent to zero, whereafter the Hall current switch may be openedwithout breaking a current. Thereafter, full power may be againdelivered to the loads when the load switches are again closed. Thus, anMHD generator may be taken off the line by the simple expedient ofmostly closing a switch rather than by the conventional procedure ofbreaking one or more high power circuits.

The arrangement described immediately hereinabove in accordance with thepresent invention is not only quite simple and effective but may beused, for example, to take an MHD generator on or 01f the line, controlits output current, or vary its output current to provide a wave formthat approaches that of a sine wave.

In view of the foregoing, it will be apparent that a broad object of thepresent invention is to provide an improved electric generator.

Another object of the present invention is to provide a means for andmethod of controlling the flow of electric current within electricalequipment.

Another object of the present invention is to provide a means for andmethod of taking electrical equipment on and off the line.

A further object of the present invention is to provide an MHD generatorwherein the Hall effect is utilized to control the generator.

A still further object of the present invention is to provide an MHDgenerator with segmented electrodes which are interconnected to permitvariation or interruption of the output current of the generator in asimple, economical, and efficient manner.

Yet another object of the present invention is to provide an MHDgenerator having pairs of opposed electrodes, each connected through aload switch to a load, the terminal electrodes of which are connected incircuit with a Hall current switch.

The novel features that I consider characteristic of my invention areset forth in the appended claims; the invention itself, however, both asto its organization and method of operation, together with additionalobjects and advantages thereof, will best be understood from thefollowing description of specific embodiments when read in conjunctionwith the accompanying drawings, in which:

FIGURE 1 is a diagrammatic representation of a conventional MHDgenerator;

FIGURE 2 is a diagrammatic representation of a novel arrangement inaccordance with the teaching of the present invention;

FIGURE 3 is a graphic representation of the manner in which the load andHall current varies for the arrangement illustrated in FIGURE 2;

FIGURE 4 is a diagrammatic representation of a modification of thearrangement illustrated in FIGURE 2;

FIGURE 5 is a graphic representation of the manner in which the Hallcurrent varies for the arrangement illustrated in FIGURE 4;

FIGURE 6 is a diagrammatic representation of another embodiment of thepresent invention;

FIGURE 7 is a graphic representation of the manner in which the currentsand voltages vary for the arrangement illustrated in FIGURE 6 when boththe load and Hall current switches are actuated;

FIGURE 8 is a graphic representation of the manner in which the currentsand voltages vary for the arrangement illustrated in FIGURE 6 when onlythe Hall current switch is actuated;

FIGURE 9 is a diagrammatic representation of another embodiment of thepresent invention for providing an AC. output; and

FIGURE 10 is a cross sectional view taken on line 10-10 of FIGURE 9.

A knowledge of the general principles of MHD generators Will promote anunderstanding of the present invention. For this reason, there is shownin FIGURE 1 a schematic of a prior art MHD generator. As illustrated inthat figure, the generator comprises a tapered duct, generallydesignated 1, to which high temperature, high pressure electricallyconductive plasma is introduced, as indicated by the arrow at 2, andfrom which it exhausts, as indicated by the arrow at 3. The pressure atthe exit of the duct is lower than at its inlet; and for this reason,the plasma moves at high velocity through the duct, as indicated by thearrow at 4. By properly choosing the pressure differential and the shapeof the duct, the plasma can be made to move through the duct atsubstantially constant velocity which is desirable, although notnecessary to the operation of the generator. Surrounding the exterior ofthe duct is a continuous electrical conductor in the form of a coil 15to which a unidirectional electrical current may be supplied from anyconventional source or from the generator itself. Flow of electricalcurrent through the coil establishes a magnetic flux through the duct,perpendicular to the direction of plasma flow and the plane of thepaper.

Within the duct are provided opposed electrodes 6 and 7. Theseelectrodes may extend along the interior of the duct parallel to thedirection of plasma movement and may be positioned opposite one anotheron an axis perpendicular to both the direction of plasma movement andthe magnetic flux. High velocity movement of the electrically conductiveplasma through the magnetic field induces a unidirectional between theelectrodes, such as indicated by the arrows at 8. The electrodes 6 and 7are connected by conductors 9 and 10 to a load 11 through whichelectrical current flows under the influence of the E.M.F. inducedbetween the electrodes.

Since electrons are lighter than ions and hence have a higher mobility,they will, in general, carry most of the current in an MHD generator.Since the forces exerted by the magnetic field are exerted on thecurrent carriers, the electrons naturally experience most of the forcesarising from their movement in the field.

As already mentioned, an electron current or conduction current isinduced between the electrodes by the cross product of the velocity ofthe plasma and the magnetic field. The magnetic field acts on thecurrent, creating a force tending to retard motion of the electronslongitudinally down the duct with the rest of the plasma. The ions, onthe other hand, being much greater in mass than the electrons, onlyexperience small forces as they move in the magnetic field and tend tobe carried downstream with the plasma. Thus, a separation of chargesoccurs, resulting in the creation of an electric field longitudinally ofthe duct.

As pointed out hereinbefore, this longitudinal field may be called theHall potential since the phenomena involved are similar to those givingrise to the so-called Hall effect observed some time ago in solidconductors. As used in the claims Hall potential means theaforementioned electric field substantially longitudinally of the ductand parallel with the direction of plasma flow which results from theaforementioned separation of charges which in turn result from the flowof conduction current.

The forces, acting on the electrons, are transmitted by them to the restof the plasma particles by collisions. Further, the movement of plasmaparticles is retarded by collision with the ions which are held by theelectric field existing between them and the upstream electrons. Inovercoming the forces resulting from collisions with the ions andelectrons, the plasma does work. This is as would be expected in adevice for generating electrical power.

In FIGURE 2 there is shown diagrammatically an MHD generator inaccordance with the present invention comprising a divergent duct 20 towhich is supplied a high temperature, high pressure plasma 21. Amagnetic field coil, indicated schematically by phantom lines at 22, isassociated with the duct 20 and provides a magnetic field perpendicularto the plane of the paper and transverse of the plasma stream. In FIGURE2, however, it will be noted that the electrodes are segmented, i.e.,the electrodes at each side of the duct 20 comprise separateelectrically insulated segment-s designated 23a, 23b, 23c, and 23d, and24a, 24b, 24c, and 24d. It will be noted that each of the electrodesegments of each group is positioned in side-by-side relationship, thegroups of segments on opposite sides of the duct defining paths forcurrent flow normal to the direction of both the plasma stream and themagnetic field flux. For convenience, this current will be referred toas the conduction cur rent. As used in the claims conduction currentmeans the aforementioned current flow substantially normal to thedirection of both the plasma stream and the magnetic field. In thegenerator shown in FIGURE 2 the conduction current is the load current.In other types of generators this may not be true.

Attention is now directed to the electrical connection of electrodesegments 23a and 24a. A load, such as, for example, an inverter 25a, isconnected through a load switch 26a to the electrode segments 23a and24a. Thus, when the load switch 26a is closed, current may be suppliedto the inverter 25a and when the load switch 26a is open, the circuit tothe inverter 25a will be broken. The inverter, which may be of aconventional type, is connected to the primary winding 27a of amulti-winding transformer, generally designated 28, having a commonsecondary 29 for delivering AC. power.

In a similar manner, electrode segments 23b and 24b are connectedthrough another load switch 26b to another inverter 25b, having atransformer primary winding 27b. The transformer primary winding 27b isalso coupled with the seconday winding 29 in time-phase relationshipwith the primary 27a. The other opposed pairs of electrode segments aresimilarly connected to separate load switches and inverters which arecoupled to the secondary winding 29, as illustrated. A Hall currentswitch 31 is connected to electrode segments 23a and 23d. Thesegmentation of the electrodes in effect completely breaks the path ofthe Hall current flow. Thus, when the Hall current switch 31 is open, nocontinuous path is provided longitudinally through the electrodesparallel to the direction of plasma flow, hence Hall currents cannotform within the plasma. In this way, the losses associated with Hallcurrents are eliminated so long as the Hall current switch 31 remainsopen. When the Hall current switch 31 is closed, a closed circuit isprovided through which Hall currents may flow during normal operation ofthe generator. The flow of Hall currents decreases direct flow of theconduction current through the plasma between the electrodes and forthis reason is normally considered to constitute a serious loss ofoperating efiiciency and, therefore, to be undesirable.

It may also be noted at this point that if the conduction current cannotflow, then the electrons in the plasma will not be retarded, and theHall potential will not appear. Obviously, if the Hall potential doesnot appear, Hall currents also cannot appear.

In order to appreciate the manner in which the present inventionoperates, assume that the Hall current switch 31 is open and the loadswitches 26a26d are closed. Under these circumstances, power will bedelivered to the inverters or any other suitable load, as in a normalMHD generator with a segmented output. When the Hall current switch 31is closed, Hall current can flow through electrode 23d, the Hall currentswitch 31, electrode 23a, and the plasma 21 between electrode 23a andelectrode 23d. As previously explained in connection with FIGURE 1, thisflow of Hall current reduces the conduction current flowing betweenopposed pairs of electrodes. If the Hall current is of sufiicientmagnitude, it will reduce the conduction current to zero. By reason ofthe reduced current flowing therethrough, the load switches 26a-26d maynow be opened with little or no arcing. If the conduction current is atsome finite value,

opening of the load switches 26a-26d reduces the conduction current tozero. As previously explained, this in turn reduces the Hall current tozero, whereafter the Hall current switch 31 may be opened withoutbreaking a current. -Full load current will again be delivered to theinverters or loads when the load switches 26a-26d are closed subsequentto the opening of the Hall current switch 31. As will now be evident,the present invention in a new and novel manner permits the variationand/or interruption of the power output of an MHD generator mostly byclosing switches rather than opening them in conventional manner.Bearing in mind that it is much more difficult to break a high powercircuit than to make it and that it is much less difficult to break alow power circuit than a high power circuit, it will immediately beapparent that the present invention is quite useful in controlling thepower output of MHD generators.

That the Hall current may be advantageously utilized to control theoutput of an MHD generator may be seen from the following discussion ofthe equations that illustrate the principles involved. It should benoted at this point, however, that the following equations aretheoretical in that they are for a uniform plasma. A uniform plasma ispresumed because it is believed that the underlying principles of thepresent invention can most clearly be explained and understood on thisbasis. However, as is always the case, theory can only be approached inactual practice. Thus, in actual practice the plasma in an MHD generatorwill not be uniform and, hence, in this respect, the equations are onlyapproximately correct.

The Hall current jx which flows parallel to the plasma stream may bedetermined from the general equation and the conduction current jy whichflows transverse of the plasma stream may be determined from the generalequation +OJ2T2['ILB where a=the scale of conductivity of the plasmaw=the electron cyclotron frequency in radians/ sec.

T=the electron mean free time between collisions with plasma particlesin seconds B=the magnetic field strength Ey=the potential betweenelectrodes transverse of the plasma stream Ex=the Hall potentiallongitudinally through the plasma stream u=the macroscopic velocity ofthe plasma stream The values for w and 1- for any given plasma can becalculated by using the principles set forth in Physics of Fully IonizedGases by Lyman Spitzer, Jr., Interscience Publishers, Inc., 1956, andother standard reference works.

Assuming no leakage effects of current through boundary layers or due toother inhomogeneities in the plasma or magnetic field, if the Hallcurrent switch 31 is open and the load switches 26a-26d are closed, theHall current jx is equal to zero and the full load current jy is givenby the following solution of the preceding general equations for ix andjy:

where 1 is the electrical efiiciency of the generator or, to state itanother way, the fraction of work done by the plasma that is deliveredto the load in the form of electrical power.

If the Hall current switch 31 is now closed, the Hall potential Exbecomes nominally equal to zero and as a result the conduction currentis reduced. The reduced conduction current j"y resulting where there isa flow of Hall current is given by the equation:

At the same time that the conduction current iy is reduced the Hallcurrent ix rises from zero to a finite value. The increased Hall currentj'x may be determined from the equation If now the load switches 26a-26dare opened, the conduction current jy will, of course, drop to zero, andit can be shown from the equations for the Hall current jx and theconduction current jy that the Hall current jx also drops to zero.

Thus, it may now be apparent that when the load switches 26a-26d areopened subsequent to the closing of the Hall current switch 31, the loadswitches will not have to break the full load current jy. Since the Hallcurrent jx drops to zero when the load switches are opened, the Hallcurrent switch when it is opened does not break a current.

Attention may now be directed to FIGURE 3, which illustrates anidealized form of load and Hall current variation for the embodimentillustrated in FIGURE 2, it being assumed that there is negligiblereactance in any of the circuits. As shown in FIGURE 3, if the loadswitches 26a-26d are closed at time t the load current, represented bythe solid line, will rise to its maximum value and remain constant untilthe Hall current switch 31 is closed. Upon closing of the Hall currentswitch at time t the load current will drop to at least a low value andremain at this value until such time as the load switches are opened,such as, for example, at time t The Hall current, represented by thebroken line, rises from zero at time t to a finite value upon closure ofthe Hall current switch 31 and, as previously indicated, drops to zeroat time t when the load switches are opened. As can readily be seen frominspection of FIGURE 3, the length of time that the Hall current ispermitted to flow is determined by the delay between the time when theHall current switch is closed and the load switches are opened. Thus,the time between t and t can be made quite short, it only beingnecessary that the opening of the load switches be delayed until theload current has reached its minimum value which, as will be pointed outhereinafter, can be zero. When the load switches are again closed attime t.,, the cycle will be repeated.

It will be seen from the previously discussed equations that theusefulness of the present invention depends upon the value of w. Thus,by way of example, for an electrical efliciency 1; of 0.8 the rat-i ofthe reduced conduction current to the full load current j"y/j'y may havethe following values for the given values of air:

or j'yljy 1 0. so JR) 0. 33 1o 0. 05

For a generator efiiciency n .of 0.5, the ratio of j"y/j'y has thefollowing values for the same values of on.

Thus, from the above it may readily be seen that if mis equal to one, areduction of only 20 to 30% in the full load current jy will beobtained. However, if M' is equal to 10, then the full load current jywill be reduced by more than a factor of 10.

In actual practice, car may be increased by increasing the magneticfield strength, decreasing the plasma density, selecting a plasmacomposition that results in a small elec tron collision cross section,or by a combination of any or allot the foregoing. Argon, for example,has an electron collision cross section roughly one-thirtieth of mostgases.

The time that Hall current flows can be made quite short. Thus, it isonly necessary that the Hall current switch be of a conventionalcontaotor type that can carry the maximum Hall current for the requiredtime since it is only necessary that it make a circuit rather than breaka circuit. F urthcr, if the conduction current is made to effectivelydrop to zero, such as, for example, in the manner hereinafter described,the load switches may also be of the contactor type, capable ofcontinuously carrying the full load current. Consider now the mostunfavorable case where, for example, for practical reasons, it may bedesired that the load switches break a current of sufficient magnitudeas to cause appreciable arcing. In this case so far as arcing isconcerned, the load switches need only be capable of breaking a currentconsiderably less than the full load current.

A word may also be said at this point with respect to ,short circuits.The Hall current and load switches may be actuated manually. They mayobviously also be actuasted in timed relationship by conventional means,such as, for example, a motor and sensing means to actuate the motor.Thus, if the sensing means is sensitive to short circuit conditions, thegenerator or circuit containing the short circuit can be immediatelyshut down thereby eliminating the necessity of considering short circuitcurrents in, for example, the selection of the load switches.

Attention is now directed to FIGURE 4, which illustrates a modificationof the embodiment shown in FIG- URE 2. The arrangement shown in FIGURE 4is identical to that shown in FIGURE 2 except for the provision of atank circuit 41 connected in series with the Hall current switch 31. Thetank circuit 41 is comprised of an inductance 42 connected in parallelwith a capacitor 43. Further, although it is not essential, it ispreferable that a rectifying action be provided in the conductioncurrent circuit or circuits. Such an effect may be achieved, forexample, by maintaining one set of electrodes at a temperature belowthat at which they will emit electrons and/or forming them of anon-emissive material. Alternately, a conventional rectifier may beprovided in the conduction current circuits, such as, for example,between opposite pairs of electrodes, if the load or loads do notfunction as a rectifier to achieve the same result.

For a sufficiently high an and a suitable choice of the values ofinductance and capacitance in the tank circuit 41, a Hall current jx dueto the action of the tank circuit will overshoot its equilibrium valuewhen the Hall current switch 31 is closed. At the same time, the conduction current jy will momentarily try to reverse its direction of flow.Thus, if the aforementioned rectifying action is provided in theconduction current circuit, then the conduction current jy willmomentarily be Zero, and the load switches may be opened at this timewithout breaking a current. The rectifying action is desirable becauseit prevents the conduction current from actually reversing, it insuresessentially zero conduction current flow, and tends to increase the timethat the conduction current is zero.

FIGURE 5 illustrates the manner in which the Hall current jx will vary.In FIGURE 5 it is assumed that the load switches are closed and the Hallcurrent switch is open. The Hall current switch is closed at time t Inorder that the conduction current drop to zero subsequent to time t itis necessary that wr and the circuit elements be chosen such that theHall current jx at time t be at least equal to and preferably greaterthan in order that the conduction current jy be reduced to zero. Therequired values of the circuit parameters will depend upon thecircumstances under which it is desired to operate and can be found bystraightforward transient circuit analysis. As has been previouslypointed out, the values of tor, the inductance, and the capacitance mustbe selected such that the initial overshoot of the Hall current at timet cuts off or materially reduces the flow of conduction current acrossthe generator duct. The time available for opening the load switches isdetermined by the relation of jx at time i to Thus, if jx at time t isjust equal to essentially zero time is available to open the loadswitches. On the other hand, the time available to open the loadswitches is increased in proportion to the amount that jx at time 1 ismade greater than The foregoing requires that anbe generally greaterthan about 4 or 5. Provision of a mgreater than about 4 or 5 greatlyfacilitates the design and operation of the load switches, since theymay be opened under zero current conditions.

Attention is now directed to FIGURE 6, which illustrates anotherarrangement for reducing the conduction current to zero. The arrangementshown in FIGURE 6 is identical to that shown in FIGURE 4 with theexception of the Hall current circuit between electrodes 23a and 23dexterior of the duct. As shown in FIGURE 6, the tank circuit of FIGURE 4is omitted, and the Hall current switch 31 of FIGURE 4 is replaced by adouble pole, double throw, reversing switch 51. The electrodes 23a and23d are connected to respectively terminals 52 and 53 of switch 51, asare switch arms 54 and 55 A capacitor 56 is connected across terminals57 and 58 of switch 51. Terminal 61 is connected to terminal 57, and theremaining terminal 62 is connected to terminal 58 in conventionalmanner, such that as switch 51 is thrown from one position to the otherthe connection of capacitor 56 to electrodes 23a and 23d is reversed.Thus, assume that the load switches 26a-26d are closed and switch 51 isopen, i.e., the Hall current circuit is open and capacitor 56 is notconnected to electrodes 23a and 23d. When switch 51 is closed in onedirection, capacitor 56 will be charged to the full Hall potential.Obviously, after the charge on capacitor 56 reaches the full Hallpotential, no further current will flow in the Hall current circuit. Ifnow switch 51 is thrown in the opposite direction, the connection ofcapacitor 56 to electrodes 23a and 23d will be reversed. Upon reversalof switch 51 the charge on capacitor 56, which is substantially equal tothe Hall potential, is connected in series aiding with the Hallpotential. Thus, a Hall current will momentarily be permitted to flowwhich may be up to twice as big as the Hall current drawn in theapparatus illustrated in FIGURE 2. For the arrangement illustrated inFIGURE 6 the value of or need not be as high as that required, forexample, by the arrangement illustrated in either FIGURE 2 or FIGURE 4.Thus, if w'r is sufliciently high, such as, for example, greater thanabout 2, the flow of Hall current resulting from reversal of switch 51will reduce the conduetion current jy to zero forthe same reasonsdiscussed in connection with FIGURE 4. Also, for the reasons discussedin connection with FIGURE 4, a rectifying action in the conductioncurrent circuits is desirable. When capacitor 56 has been charged up inthe reverse direction due to the reversal of switch 51, returning switch51 to its original position will produce another momentary cutoff of theconduction current jy. The duration of cutoif of the conduction currentjy is determined by the Values of the capacitor 56, a, and the loadimpedances. Again, the required values of circuit parameters for aspecific application can be found by straightforward transient circuitanalysis.

Assuming resistive circuits, FIGURE 7 illustrates the manner in whichthe conduction current jy, Hall current ix, and Hall potential Ex willvary for the various positions of the Hall current switch 51 and theload switches 26a-26d discussed immediately hereinabove. Thus, uponreversal of the connection of capacitor 56 to electrodes 23a and 23d byactuation of the Hall current switch 51 from one position to another attime t the conduction current jy will tend to reverse as suggested bythe negative portion of the curve jy. The presence of a rectifyingaction will eliminate this negative portion. Simultaneously at time tthe Hall current ix will increase from zero to its maximum value andthereafter decrease at a rate determined by the charge on capacitor 56and the magnitude of the Hall potential Ex between electrodes 23a and23d. Also, at time t; the Hall potential Ex drops from its normal orsteady state value to a negative value and thereafter increases in apositive direction. If now the load switches 26a-26d are opened at timet the load current is fixed at zero, and the Hall current ix dropsrapidly as shown due to the removal of the Hall potential. The Hallcurrent jx that flows after the load switches are opened is due to andcontrolled by the charge on capacitor 56. The Hall potential Ex, ofcourse, continues to rise from a negative value toward zero, and itsrate of rise is determined by the flow of Hall current.

It the load switches 26a-26d are closed at time t the conduction currentjy and the Hall potential Ex rapidly rise to their steady state values.Thus, the flow of Hall current jx through capacitor 56 rapidly decreasesfrom a value less than its value at time t The curves of voltage andcurrent illustrated in FIG- URE 8 show the relation and manner ofvariation of the conduction current jy, Hall current jx, and Hallpotential Ex for reversal of the Hall current switch 51 withoutactuation of the load switches 26a26d.

Inspection of the curve for the conduction current jy in FIGURE 8 willshow that if, for example, the polarity of the conduction current isalternately reversed with respect to the load during the time betweenabout time i and t a current output waveform may be obtained thatapproaches that of a sine wave. This may be accomplished, for example,by use of an electrode arrangement and output circuit describedhereinafter in connection with FIGURE 9.

With reference now to FIGURE 9, there is shown an arrangement similar tothat illustrated in FIGURE 6. However, several important differencesshould be noted. Firstly, it should be noted that the electrodes foraccommodating conduction current are comprised of two groups ofoppositely disposed electrodes designated generally by the numbers 71and 72. Group 71 is comprised of electrodes 73a73c and 74a-74c and group72 is comprised of electrodes 75a-75c and 7611-760.

Next, it should be noted that separate terminal electrodes, designatedby the numbers 77, 78, and 79, comprised of a pair of oppositelydisposed electrodes parallel to the magnetic field are associated withthe groups of electrodes 71 and 72. The terminal electrodes 77, 78, and79 accommodate the flow of Hall current. Directing attention to FIGURE10, it will be noted that the terminal electrode 77 may be comprised ofa pair of interconnected electrode segments 81 and 82 carried inrespectively op posite sidewalls 83 and 84 of the duct perpendicular tothe magnetic field. Thus, electrode segments 81 and 82 are positionedparallel to the magnetic field and perpendicular to the direction ofplasma flow. Terminal electrodes 78 and 79 may be essentially identicalto terminal electrode 77. Terminal electrode 77 is positioned upstreamof electrodes 73a and 7411, and terminal electrode 79 is positioneddownstream of electrodes 75c and 760. Terminal electrode 78, however, ispositioned intermediate electrodes 73c, 74c, and 75a, 76a. Terminalelectrode 78 thus separates the electrodes for accommodating conductioncurrent into two group-s as and for the purposes hereinafter described.All of the electrodes, whether for accommodating conduction current orHall current, are insulated from the duct. Further, the terminalelectrodes may be comprised of a greater number of electrode segmentsthan that shown or of annular rings suitably supported within andinsulated from the duct. However, where a ring-type terminal electrodeor its equivalent is used, it must be positioned along a plane of equalpotential within the generator to prevent internal shorting of thegenerator. A-complete discussion of such terminal electrodeconfigurations may be found in my patent application Serial Number32,969, filed May 5, 1960, to which reference is made.

Next, it should be noted that the Hall current reversing switch 51 ofFIGURE 6 is replaced by a single pole, double throw Hall current switch85 having three terminals, 86, 87, and 88. Terminal 87 of the singlepole, double throw Hall current switch 85 is connected to theintermediate terminal electrode 78 through a capacitor 89 and conductor91. Terminal 86 is connected through conductor 92 to terminal electrode77, and terminal 88 is connected through conductor 93 to terminalelectrode 79. Electrodes 73a and 74a are connected through a load switch94a to a primary winding 95a of an output circuit comprising amuiti-winding transformer, generally designated 96, having common asecondary winding 97 for delivering A.C. power. In a similar manner,electrode segments 73b and 74b are connected through another load switch94b to another primary winding 95b. The primary winding 95]; is alsocoupled with the secondary winding 97 in time-phase relationship withthe primary winding 95a. The other opposed pairs of electrode segmentsare similarly connected to separate load switches and primary windings,as illustrated. The dots associated with the primary windings 9511-95cand 9911- 990 of transformer 96 indicate reversal of the polarity ofthese windings. Thus, a current induced in the secondary winding 97 byprimary windings 9511-950 will be opposite in polarity to that inducedin the secondary winding by primary windings 9911-990. The connection ofthe Hall current and load switches to suitable actuating means,designated by the number 101, suggests that these switches may beactuated in timed relationship one with another.

The principle of operation of the arrangement illustrated in FIGURE 9 issimilar to that described in connection with FIGURE 6. Thus, assume thatload switches 9411-940 are closed, that load switches 9811-980 are open,that the Hall cunrent switch 85 is connected between terminals 87 and88, and that there is no charge on capacitor 89. If the load switches9811-980 are now closed, a charge will build up on capacitor 89 at thesame time that the current through load switches 9811-98c increases. Ifthe Hall current switch 85 is now thrown to its other position orterminal 86, capacitor 89 will be connected between the intermediateterminal electrode 78 and terminal electrode 77 and the charge oncapacitor 89 will be connected in series aiding with the Hall potentialbetween the aforementioned terminal electrodes. Thus, a Hall currentwill be permitted to flow, for example, between terminal electrodes 77and 78, which may be up to twice as big as the Hall current that wouldbe drawn if terminal electrodes 77 and 78 were merely short circuited.Due to the flow of Hall current between the aforementioned terminalelectrodes, the conduction current between electrodes 7311-730 and7411-74c is reduced. Load switches 94a-94lc are now opened. The loadswitches 946l946 may remain open, for example, for about one-half cycleof the desired period of the alternating current output. The loadswitches 9411-940 are then closed and a charge builds up on capacitor 89due to the potential gradient between terminal electrodes 77 and 78. Thepolarity of this new charge on capacitor 89 will be reversed from thatof the previous charge. After the potential on capacitor 89 has reacheda suitable level, the Hall current switch is thrown from terminal 86 toterminal 88 to reduce the current flowing through load switches9811-980. Thereafter, load switches 980-986 may be opened to repeat thecycle. The current induced in the secondary winding 97 when loadswitches 98a-98c are closed is out of phase with the current previouslyinduced therein by the flow of current in primary windings 9511-950.Although a limited amount of overlap is present in the flow of currentthrough the primary windings, it will now be evident that an alternatingcurrent output is available at the secondary winding.

In view of the preceding discussion, it will be apparent that theelectrode arrangement and output circuit illustrated in FIGURE 9 may bemodified to utilize the switching arrangement illustrated in FIGURE 2 orFIG- URE 4.

It will now be obvious that the present invention is subject to variousmodifications and may be utilized in a number of different ways. Forexample, the present invention may be utilized to take an MHD generatoron or off the line, vary its output current, or produce alternatingcurrent. In short, it may be used to control an MHD generator. Further,separate terminal electrodes of various construction may be utilized orcertain of the electrodes for accommodating conduction current may alsofunction as terminal electrodes. Electrode arrangements foraccommodating conduction current may be repeated to provide, forexample, separate load circuits or an alternating current output. Theload switches may be actuated in timed relationship with the Hallcurrent switchfor certain purposes, or the Hall cunrent switch alone maybe actuated. Also, the electrodes for accommodating conduction currentneed not necessarily be constructed or connected as shown and describedherein. The present invention is equally useful with electrodearrangements, for example, as shown and described in the patentapplications to which reference has previously been made. In thesecases, where it is desired, for example, to take an MHD generator on oroff the line, the Hall current switch would function to short out theload, contrary to accepted practice or what one would expect. Also, whenthe electrodes for accommodating conduction current are connected inseries, the number of load switches need not necessarily equal thenumber of pairs of oppositely disposed electrodes.

The various features and advantages of the invention are thought to beclear from the foregoing description. Various other features andadvantages not specifically enumerated will undoubtedly occur to thoseversed in the art, as likewise will many variations and modifications ofthe embodiments of the invention illustrated, all of which may beachieved without departing from the spirit and scope of the invention asdefined by the following claims.

1. In combination, first means for conveying an electrically conductivefluid; means for establishing magnetic flux through said first means atan angle to the direction of flow of the buid; a plurality of discreteelectrodes spaced from each other within said first means, theconnection in circuit of said plurality of electrodes allowing apotential gradient within the fluid in the direction of flow; and meansfor selectively reducing said potential gradient to about zero.

2. In combination, first means for conveying an electrically conductviefluid; means for establishing magnetic flux through said first means atan angle to the direction of flow of the fluid; a plurality of discreteelectrodes spaced from each other within said first means, theconnection in circuit of said plurality of electrodes allowing apotential gradient within the fluid in the direction of flow; and meansfor selectively providing substantially a short circuit of saidpotential gradient.

3. In combination, first means for conveying an electrically conductivefluid; means for establishing magnetic flux through said first means atan angle to the direction of flow of the fluid; a plurality of discreteelectrodes spaced from each other within said first means, the flow ofcurrent through said plurality of electrodes allowing a potentialgradient within the fluid in the direction of flow; and means forselectively providing substantially a short circuit of said potentialgradient for controlling the current flow through said plurality ofelectrodes.

4. In a device for generating electric power wherein amagnetohydrodynamic generator employs an electrically conductive fluidhaving a potential gradient within the fluid in the direction of flow,the combination with said generator of means for selectively providingsubstantially a short circuit of at least a portion of said potentialgradient.

5. In a device for generating electric power wherein amagnetohydrodynamic generator employs an electrically conductive fluidhaving a potential gradient within the fluid in the direction of flow,the combination with said generator of means including switching meansfor providing substantially a short circuit of at least a portion ofsaid potential gradient.

6. In a device for generating electric power wherein amagnetohydrodynamic generator employs an electrically conductive fluidhaving a potential gradient within the fluid in the direction of flow,the combination with said generator of means for selectively providingsubstantially a short circuit external of said fluid of at least aportion of said potential gradient, said means including switchingmeans.

7. In a device for generating electric power wherein amagnetohydrodynamic generator employs a duct for conveying a stream ofelectrically conductive fluid having a potential gradient in itsdirection of flow, means for establishing a magnetic flux through saidduct normal to the direction of flow of said fluid, and opposedelectrodes within said duct aligned perpendicularly to the magnetic fluxand the direction of flow of the fluid, the combination with saidgenerator of means for selectively providing external of said fluidsubstantially a short circuit of at least a portion of said potentialgradient.

8. In a device for generating electric power wherein amagnetohydrodynamic generator employs a duct for conveying a stream ofelectrically conductive fluid having a potential gradient in itsdirection of flow, means for establishing a magnetic flux through saidduct normal to the direction of flow of said fluid, and opposedelectrodes within said duct aligned perpendicularly to the magnetic fluxand the direction of flow of the fluid, the combination with saidgenerator of means including first switching means for selectivelyproviding external of said fluid substantially a short circuit of atleast a portion of said potential gradient; and second switching meansconnected in series with certain of said opposed electrodes.

9. In a device for generating electric power wherein amagnetohydrodynamic generator employs a duct for conveying a stream ofelectrically conductive fluid having a potential gradient in itsdirection of flow, means for establishing a magnetic flux through saidduct normal to the direction of flow of said fluid, and opposedelectrodes within said duct aligned perpendicularly to the magnetic fluxand the direction of flow of the fluid, the combination with saidgenerator of means including first switching means for selectivelyproviding external of said fluid substantially a short circuit of atleast a portion of said potential gradient; second switching meansconnected in series with at least two opposed electrodes; and means foractuating said first and second switching means in timed relationship.

10. In a device for generating electric power wherein amagnetohydrodynamic generator employs a duct for conveying a stream ofelectrically conductive fluid having a potential gradient in itsdirection of flow, means for establishing a magnetic flux through saidduct normal to the direction of flow of said fluid, and opposedelectrodes within said duct aligned perpendicularly to the magnetic fluxand the direction of flow of the fluid, the combination with saidgenerator of means including first switching means for selectivelyproviding external of said fluid substantially a short circuit of atleast a part of said potential gradient; second switching meansconnected in series wih selected pairs of opposed electrodes; and meansfor actuating said first and second switching means in timedrelationship.

11. In combination, first means for conveying an electrically conductivefluid; means for establishing magnetic flux through said first means atan angle to the direction of flow of the fluid; a plurality of discreteand opposed electrodes spaced from each other within said first meansand connected in circuit, the connection in circuit of opposedelectrodes establishing a potential gradient within the fluid in thedirection of flow; means for selectively providing substantially a shortcircuit of at least a portion of said potential gradient; and switchingmeans con nected in series with selected pairs of opposed electrodes.

12. In combination, first means for conveying an electrically conductivefluid; means for establishing magnetic flux through said first means atan angle to the direction of flow of the fluid; a plurality of discreteelectrodes spaced from each other within said first means, theconnection in circuit of said plurality of electrodes establish ing apotential gradient within the fluid in the direction of flow; firstmeans for selectively providing substantially a short circuit ofditferent portions of said potential gradient; switching means connectedin series with selected pairs of opposed electrodes; and second meansfor actuating said first means and said switching means in timedrelationship.

13. In a magnetohydrodynamic generator the combination comprising: aduct for conveying a stream of electrically conductive plasma; firstmeans for establishing a magnetic flux through said duct substantiallynormal to the direction of flow of the plasma; opposed electrodes withinsaid duct and aligned perpendicularly to the magnetic flux and thedirection of flow of the plasma, each of said electrodes comprisingdiscrete segments whereby current flow substantially perpendicular tothe magnetic flux and the direction of flow of the plasma between theopposed segments establishes a potential gradient within the plasma inthe direction of flow; second means including switching means forcausing said potential gradient to at least momentarily decrease; andcircuit interrupting means connected in series with selected pairs ofopposed electrodes.

14. In a magnetohydrodynamic generator the combination comprising: aduct for conveying a stream of electrically conductive plasma; firstmeans for establishing a magnetic flux through said duct substantiallynormal to the direction of flow of the plasma; opposed electrodes withinsaid duct and aligned perpendicularly to the magnetic flux and thedirection of flow of the plasma, each of said electrodes comprisingdiscrete segments whereby current flow substantially perpendicular tothe magnetic flux and the direction of flow of the plasma between theopposed segments establishes a potential gradient within the plasma inthe direction of flow; second means including switching means forpermitting at least momentary current flow within said plasma parallelto the direction of plasma flow; and circuit interrupting meansconnected in series with selected pairs of opposed electrodes.

15. In a magnetohydrodynamic generator the combination comprising: aduct for conveying a stream of electrically conductive plasma; firstmeans for establishing a magnetic flux through said duct substantiallynormal to the direction of flow of the plasma; opposed electrodes withinsaid duct and aligned perpendicularly to the magnetic flux and thedirection of flow of the plasma, each of said electrodes comprisingdiscrete segments whereby conduction current flow between the opposedsegments establishes a Hall potential within the plasma in the directionof flow; second means including switching means for permit-tingsufficient Hall current to flow at least momentarily to reduceconduction current; and circuit interrupting means connected in serieswith selected pairs of opposed electrodes to open circuit the conductioncurrent.

16. The combination as defined in claim 15 wherein the second meansadditionally includes a tank circuit.

17. The combination as defined in claim 15 wherein the second means iscoupled to different portions of the Hall potential.

18. The combination as defined in claim 15 wherein said second means isconnected between terminal electrode segments.

19. The combination as defined in claim 15 additionally includingterminal electrodes, said terminal electrodes separating the electrodesegments into groups, said second means being coupled to said terminalelectrodes; and means for actuating said second means and said circuitinterrupting means in timed relationship.

'20. In a magnetohydrodynamic generator the combination comprising: aduct for conveying a stream of electrically conductive plasma; firstmeans for establishing a magnetic flux through said duct substantiallynormal to the direction of flow of the plasma; opposed electrodes withinsaid duct and aligned perpendicularly to the magnetic flux and thedirection of flow of the plasma, each of said electrodes comprisingdiscrete segments whereby conduction current flow between the opposedsegments establishes a Hall potential within the plasma in the directionof flow; second means for permitting Hall current to flow, said secondmeans including a source of potential and means for connecting saidsource of potential in series aiding across at least a selected portionof said Hall potential to reduce conduction current within said selectedport-ion to about Zero; and second circuit interrupting means connectedin series with selected pairs of opposed electrodes.

21. The combination as defined in claim wherein said source of potentialis a capacitor and said means for connecting said source of potential inseries aiding is a reversing switch.

22. The combination as defined in claim 20 wherein said second meansincludes a source of potential and means for connecting saidsource ofpotential in series aiding across different portions of said Hallpotential for reducing conduction current within said dilferent portionsto about Zero; and additionally including means for actuating saidsecond means and circuit interrupting means in timed relationship.

23. In a device for generating electric power wherein amagnetohydrodynamic generator which generates electrical power bymovement of electrically conductive fluid relative to magnetic fieldemploys a plurality of discrete electrodes spaced from each other, thecombination with said generator of means for selectively providingsubstantially a short circuit between certain of the electrodeslongitudinally of the generator.

24. The method of controlling the flow of conduction current within astream of plasma flowing through a magnetic field at an angle to thedirection of flow of the plasma comprising: selectively causing currentto flow longitudinally through the plasma.

25. The method of controlling the flow of conduction current within astream of plasma flowing through a magnetic field at an angle to thedirection of flow of the plasma and having a Hall potential in thedirection of flow comprising: selectively permitting Hall current toflow longitudinally through the plasma.

26. The method of controlling the flow of conduction current within astream of plasma flowing through a magnetic field at an angle to thedirection of flow of the plasma and having a Hall potential in thedirection of flow comprising: selectively short circuiting the Hallpotential.

27. The combination as defined in claim 26 and additionally includingthe step of interrupting the conduction current subsequent to shortcircuiting of the Hall potential.

28. The method of controlling the flow of conduction current within astream of plasma flowing through a magnetic field at an angle to thedirection of flow of the plasma and having a Hall potential in thedirection of flow comprising: selectively substantially short circuitingthe Hall potential at least momentarily through a source of potentialconnected in series aiding with said Hall potential.

29. The method of controlling a magnetohydrodynamic generator having aduct through which flows a stream of electrically conductive plasma,means for establishing magnetic flux through the plasma perpendicularlyto its direction of flow, and opposed segmented electrodes within theduct between which conduction current flows substantialy mutuallyperpendicular to the direction of plasma flow and the magnetic flux, aHall potential normally existing within the plasma in the direction offlow comprising: substantialy short circuiting at least a portion ofsaid Hall potential; thereafter interrupting the flow of conductioncurrent disposed within said short circuited portion of the Hallpotential; and thereafter removing said short circuit.

30. The method of controlling a magnetohydrodynamic generator having aduct through which flows a stream of electrically conductive plasma,means for establishing magnetic flux through the plasma perpendicularlyto its direction of flow and opposed segmented electrodes within theduct between which conduction current flows substantially mutuallyperpendicular to the direction of plasma flow and the magnetic flux, aHall potential normally existing within the plasma in the direction offlow comprising: causing a current to flow longitudinally through atleast a portion of the plasma sufiicient to at least reduce conductioncurrent in said portion; and interrupting said conduction current insaid portion of the plasma when the longitudinal current flow is aboutmaximum.

References Cited by the Examiner UNITED STATES PATENTS 2,592,970 9/60Blackman 310-11 FOREIGN PATENTS 841,613 6/52 Germany. 1,161,079 3/58France.

MILTON O. HIRSHFIELD, Primary Examiner.

DAVID X. SLINEY, Examiner.

1. IN COMBINATION, FIRST MEANS FOR CONVEYING AN ELECTRICALLY CONDUCTIVEFLUID; MEANS FOR ESTABLISHING MAGNETIC FLUX THROUGH SAID FIRST MEANS ATAN ANGLE TO THE DIRECTION OF FLOW OF THE BUID; A PLURALITY OF DISCRETEELECTRODES SPACED FROM EACH OTHER WITHIN SAID FIRST MEANS, THECONNECTION IN CIRCUIT OF SAID PLURALITY OF ELECTRODES ALLOWING APOTENTIAL GRADIENT WITHIN THE FLUID IN THE DIRECTION OF FLOW; AND MEANSFOR SELECTIVELY REDUCING SAID POTENTIAL GRADIENT TO ABOUT ZERO.